Natural gas is a fuel gas used extensively in the petrochemical and other chemicals businesses. Natural gas is comprised of light hydrocarbons-primarily methane, with smaller amounts of other heavier hydrocarbon gases such as ethane, propane, and butane. Natural gas may also contain some quantities of non-hydrocarbon “contaminant” components such as carbon dioxide and hydrogen sulfide.
Natural gas is often extracted from natural gas fields that are remote or located off-shore. Conversion of natural gas to a liquid hydrocarbon is often required to produce an economically viable product when the natural gas field from which the natural gas is produced is remotely located with no access to a gas pipeline. One method commonly used to convert natural gas to a liquid hydrocarbon is to cryogenically cool the natural gas to condense the hydrocarbons into a liquid. Another method that may be used to convert natural gas to a liquid hydrocarbon is to convert the natural gas to a synthesis gas by partial oxidation or steam reforming, and subsequently converting the synthesis gas to a liquid hydrocarbon, such as that produced by a Fisher-Tropsch reaction. Synthesis gas prepared from natural gas may also be converted to a liquid hydrocarbon oxygenate such as methanol.
Non-hydrocarbon components of natural gas are generally considered contaminants when converting the natural gas to a liquid hydrocarbon or a liquid hydrocarbon oxygenate. Carbon dioxide is one such non-hydrocarbon contaminant, particularly when present in the natural gas in large quantities.
In a cryogenic cooling process to liquefy hydrocarbons in a natural gas, carbon dioxide may crystallize when cryogenically cooling the natural gas, blocking valves and pipes used in the cooling process. Further, carbon dioxide utilizes volume in a cryogenically cooled liquid hydrocarbon/carbon dioxide mixture that would preferably be utilized only by the liquid hydrocarbon, particularly when the liquid hydrocarbon is to be transported from a remote location.
Carbon dioxide also may impair conversion of natural gas to synthesis gas so that a liquid hydrocarbon or a liquid hydrocarbon oxygenate cannot be prepared by converting the natural gas to a synthesis gas and subsequently converting the synthesis gas to a liquid hydrocarbon or liquid hydrocarbon oxygenate. Significant quantities of carbon dioxide may impair conversion of a natural gas to synthesis gas by either partial oxidation or by steam reforming.
Partial oxidation of the natural gas to produce synthesis gas is usually effected by combustion of the natural gas with an oxygen containing gas at high temperatures-typically at least 700° C. when the partial oxidation is catalytically induced and at least 900° C. when the partial oxidation is effected with no catalyst. If a significant amount of carbon dioxide is present in the natural gas, the carbon dioxide tends to quench the combustion, limiting the effectiveness of the partial oxidation reaction to produce synthesis gas from the natural gas. Additionally, further processing a hot synthesis gas product produced by partial oxidation involves significant heat transfer and loss of thermal energy since the synthesis gas produced by partial oxidation must be cooled by at least 400° C., typically at least 500° C. to 700° C., prior to its utilization to produce a liquid hydrocarbon or a liquid hydrocarbon oxygenate, and much thermal energy is lost in such heat transfers. Thermal energy loss from partial oxidation of a natural gas containing large quantities of carbon dioxide at temperatures exceeding 700° C. is particularly excessive since the large volume of carbon dioxide present in the natural gas must be extensively cooled after partial oxidation as well as the synthesis gas product.
Highly active partial oxidation catalysts, e.g. those disclosed in Applied Catalysis A: General, Volume 292, 18 Sep. 2005, pp. 177-188 consisting of rhodium or ruthenium on a carrier, may be used to effect a catalytic partial oxidation of methane or natural gas at lower temperatures, for example, from 350° C. to 700° C. Catalytic partial oxidation at these lower temperatures with these catalysts is disclosed to generate carbon dioxide as a product, which is undesirable when the starting feed material is already highly contaminated with carbon dioxide. Further, auto-ignition of a hydrocarboneous feed and an oxygen containing gas would be expected to be quenched by high levels of carbon dioxide at such low temperatures.
Steam reforming natural gas to produce synthesis gas is an endothermic process, unlike partial oxidation, and requires input of heat to drive the reaction. If a significant amount of carbon dioxide is present in the natural gas, the heat duty required to produce the synthesis gas is large since heat must be supplied to heat the carbon dioxide as well as the methane and steam reactants. Further, the carbon dioxide acts as a diluent, reducing the rate of the steam reforming reaction by reducing the interaction of the methane and water molecules. Steam reforming, like partial oxidation, involves significant heat transfer and loss of thermal energy to reduce the temperature of the synthesis gas product prior to its utilization to produce a liquid hydrocarbon or a liquid hydrocarbon oxygenate due to the high temperatures at which steam reforming must be effected-typically from 700° C. to 1000° C.
As a result of the difficulty of processing natural gas contaminated with carbon dioxide, carbon dioxide present in a carbon dioxide contaminated natural gas is generally separated from the hydrocarbon components of the natural gas prior to processing the natural gas to a liquid. Separation techniques include scrubbing the natural gas with a liquid chemical, e.g. an amine, to remove carbon dioxide, passing the natural gas through molecular sieves selective to separate carbon dioxide from the natural gas, and passing the natural gas through a membrane selective to separate carbon dioxide from the natural gas. These methods of separating carbon dioxide from a natural gas are effective for natural gases containing 40 vol. % or less of carbon dioxide, more typically 20 vol. % or less, but are either ineffective or commercially prohibitive in energy costs to separate carbon dioxide from natural gas when the natural gas is contaminated with at least 50 vol. % of carbon dioxide.
Production of natural gas from natural gas fields containing natural gas contaminated with at least 50 vol. % carbon dioxide is generally not undertaken due to the difficulty of producing liquid hydrocarbons or liquid hydrocarbon oxygenates from natural gas contaminated with such large quantities of carbon dioxide and the difficulty of removing carbon dioxide from the natural gas when present in such a large quantity. However, some of the largest natural gas fields discovered to date are contaminated with high levels of carbon dioxide. Therefore, there is a need for an energy efficient, effective method to produce liquid hydrocarbons or liquid hydrocarbon oxygenates from a natural gas highly contaminated with carbon dioxide.
U.S. Pat. No. 6,702,960 provides a process for the catalytic partial oxidation of a hydrocarbonaceous feedstock to produce a synthesis gas. The hydrocarbonaceous feedstock may be methane, natural gas, or other sources of light hydrocarbons, and optionally may contain up to 60 vol. % carbon dioxide, especially 0.1-40 vol. % carbon dioxide. Although the process provides that a hydrocarbonaceous feed containing greater than 50 vol. % carbon dioxide (e.g. up to 60 vol. %), may be partially oxidized the partial oxidation is effected at a temperature of from 750° C. to 1400° C., which, as described above, is inefficient energetically due to loss of thermal energy in heat transfers, requires large heat exchangers formed of materials specifically constructed to withstand very high temperatures, and is subject to inefficient operation due to high levels of carbon dioxide quenching the combustion.
Improved processes are desirable for processing a natural gas contaminated with greater than 50 vol. % carbon dioxide to enable production of liquid hydrocarbons or liquid hydrocarbon oxygenates from natural gas fields containing high levels of carbon dioxide. In particular, more energy efficient processes that are effective for converting a natural gas contaminated with at least 50 vol. % carbon dioxide to a synthesis gas and subsequently to a liquid hydrocarbon or a liquid hydrocarbon oxygenate are desirable.